This invention relates generally to using spectroscopic techniques for gas concentration measurements, and in particular to using a ratio of gas concentrations to obtain temperature-independent measurements.
Gas concentrations are measured in laboratory experiments, industrial plant operations as well as monitoring and sensing in public or private areas. In some of these cases the measurements are performed to determine what chemical reactions are taking place, in others they represent direct results, e.g., concentrations of pollutants in the atmosphere such as exhausts from stack and vehicles (smog check and on-road remote sensing).
Gas concentration sensors based on spectroscopic techniques and absorption spectroscopy in particular have been widely used for many industrial applications as well as vehicle exhaust monitoring. For general information on the use of absorption spectroscopy, including IR and UV absorption for monitoring vehicle emissions the reader is referred to: Bishop G. A., et al., IR long-path photometry: a remote sensing tool for automobile emissions, Anal. Chem. A, 1989, Vol. 61, pp. 671-77; Cadle S. H., et al., Remote sensing of vehicle exhaust emissions, Environ. Sci. Technol. A, 1994, Vol. 28, pp. 258-64; Stephens R. D., et al., Remote sensing measurements of carbon monoxide emissions from on-road vehicles, J. Air Waste Management Assoc., 1991, Vol. 41, pp. 39-46. In addition, U.S. Pat. No. 5,498,872 to Stedman et al. teaches an apparatus for remote analysis of vehicle emissions in vehicle exhaust including, for example, concentrations of CO, CO2, HC, NO and H2O by using wide-band radiation.
The above-mentioned references take advantage of known absorption techniques which measure gas concentrations by monitoring the attenuation of optical radiation passing through the sample containing the probe gas. Attenuation of the optical radiation is due to optical radiation getting absorbed at wavelengths corresponding to certain transitions in the molecules of the probe gas. In other words, when the incident radiation contains photons at wavelengths corresponding to absorbing transitions, also referred to as spectroscopic transitions of the probe gas molecules, then some of these photons will be absorbed by the probe gas molecules. The attenuation is generally proportional to the amount of the probe gas molecules encountered by the radiation along its path. In addition, the amount of attenuation suffered by optical radiation passing through a sample of the probe gas is dependent on the gas temperature and the gas mixture composition of the sample. That is because these parameters affect the linestrength and linewidth of the selected transition or transitions. In order to correct for these effects prior art methods require temperature and gas composition information.
Unfortunately, in many situations the temperature and gas composition data required by prior art methods to correct for linestrength and linewidth is unknown. In other cases, gas composition and/or temperature measurements are not feasible or difficult. In the example of on-road remote sensing of vehicle exhausts, the temperature and composition distributions along the optical beam path are non-uniform and unknown. Similar problems are encountered in monitoring emissions from stacks, especially into turbulent and hence non-uniform atmosphere.
These limitations lead to errors of traditional absorption spectroscopy techniques in determining concentrations of probe gases. These errors are especially large in samples exhibiting large non-uniformities and/or significant fluctuations of temperature and composition profiles along the probe beam path. It would be an advance in the art to provide a technique for measuring a concentration of a probe gas in a sample without the need to determine the temperature and gas composition along the path of the probe beam of the spectrometer.
In view of the above, it is an object of the invention to provide a spectroscopic method for accurately determining the concentration of a probe gas in a sample without the necessity to determine the temperature and gas composition of the sample. Specifically, the method of the invention does not require knowledge of the temperature and gas composition along the path of the probe beam for line-of-sight absorption spectroscopy techniques. For point measurement spectroscopy such as laser induced fluorescence, the method does not require knowledge of the temperature and composition at that point.
It is another object of the invention to ensure that the method of the invention can be practiced in monitoring emissions in environments which are uncontrolled and in environments where gas temperature and composition are unknown. Specifically, it is an object of the invention to adapt the method for monitoring emissions from stacks and exhaust emissions from vehicles such as cars and airplanes.
Yet another object of the invention is to provide an apparatus for practicing the method of the invention.
These as well as other objects and advantages will become apparent upon review of the following detailed description.
The objects and advantages of the invention are achieved by a method for temperature-independent determination of a concentration of a probe gas in a sample. First, a temperature range is selected. Preferably, the temperature range extends from a low temperature TL corresponding to a lowest temperature expected or found in the sample and a high temperature TH corresponding to a highest temperature expected or found in the sample. Next, a first spectroscopic technique is selected and a probe temperature function of the probe gas is determined over the temperature range using that first spectroscopic technique. Then, a second spectroscopic technique is selected and a reference gas is identified. A reference temperature function of the reference gas is determined using the second spectroscopic technique over the temperature range. It should be noted that the first and second spectroscopic techniques can be the same. The reference gas is identified such that a ratio of the probe temperature function and the reference temperature function is substantially constant over the temperature range. For example, the ratio of the temperature functions can be substantially equal to one over the temperature range. A probe reaction of the probe gas and a reference reaction of the reference gas is then measured by the first and second spectroscopic techniques and the concentration of the probe gas is derived from the probe reaction and reference reaction.
In one embodiment either one or both of the spectroscopic techniques are absorption spectroscopy employing a test beam. The test beam consists of light at several wavelengths with at least one wavelength for either probe transition or reference transition. The test beam passing through the sample causes the probe gas and the reference gas to absorb wavelength components of the light of the test beam corresponding to the probe and reference transitions. In other words, probe reaction is a probe absorption of a wavelength component corresponding to a probe absorption transition used for detecting the probe gas. Similiarly, reference reaction is a reference absorption of a wavelength component corresponding to a reference absorption transition used for detection of the reference gas.
At the detection side, the light of the test beam is separated by wavelength using appropriate optical components. For example, different wavelength components can be split and directed to separate photodetectors. The attenuations corresponding to the probe and reference transitions can then be obtained from signals detected at the different photodetectors.
The absorptive transitions of the probe gas and reference gas at which absorption occurs can be selected from any suitable transitions. For molecular gas species the transitions can be selected from rotational, rovibrational and rovibronic transitions. For atomic gas species the transitions are electronic transitions. Selection of these transitions can be based on a vector difference between the ratio of the probe and reference temperature functions and a constant value. Preferably, the transitions for which the smallest vector difference is obtained are selected to thus minimize the temperature effect.
In the same or another embodiment the spectroscopic techniques use broadband light sources spanning broadband spectra. In this case the overall absorption of light by the probe gas and the reference gas can be measured. The reference temperature function in this embodiment can be obtained from a linear combination of a first reference temperature function in a first portion of the broadband spectrum and a second reference temperature function in a second portion of the broadband spectrum. For example, the first reference temperature function is associated with measurements with a first broadband source and the second reference temperature is associated with the second broadband source. Such linear combination can be used if the reference temperature function does not yield a sufficiently constant ratio of probe and reference temperature functions. Similarly, the probe temperature function can also be obtained from a linear combination of a first probe temperature function associated with the first broadband source and a second probe temperature function associated with the second broadband source.
In another embodiment the spectroscopic techniques employ narrowband sources having narrow spectral widths. In particular, the spectral widths are preferably narrower then the corresponding transition linewidths in the probe and reference gases. The probe reaction and reference reaction can be associated with corresponding transitions in the probe and reference gases. If the reference temperature function fails to yield a sufficiently constant ratio of probe and reference temperature functions a linear combination of functions can be employed. In particular, the reference reaction can be associated with a first transition and a second transition. The reference temperature function is now obtained from a linear combination of a first reference temperature function of the first transition and a second temperature function of the second transition.
When using a narrowband source, its spectral width is preferably narrower, even much narrower than the linewidths of the reference transitions. In the case of using two transitions in the probe gas in a linear combination, the spectral width should be narrower than the linewidth of each of these. The probe reaction can be an attenuation resulting from a probe transition and the spectral width of the narrowband source should be narrower than the linewidth of that probe transition.
In deriving the concentration of the probe gas it is convenient to derive a measured concentration ratio of the probe gas to said reference gas. This measured concentration ratio can be obtained with the aid of known techniques from the probe and reference reactions, e.g., amount of absorption by the transitions in the probe and reference gases.
The probe and reference temperature functions can be indexed to a reference temperature Tref. Although Tref can be selected outside the temperature range, it is convenient that Tref be chosen within the temperature range such that TLxe2x89xa6Trefxe2x89xa6TH.
The selection of reference gas in the sample can be based, in addition to the conditions discussed above, on other characteristics of the gas. It is convenient to choose as reference gas one that is stable and has a functional relationship with the probe gas. The functional relationship can be a well-established relation between the gas concentrations under some known conditions. This functional relationship can then be used in deriving the concentration of the probe gas. For example, in one embodiment, the probe gas is CO and the reference gas is CO2. A sample containing these two can be a vehicle exhaust sample.
The method of the invention is particularly well-suited for determining probe gas concentrations in samples which exhibit non-uniformities. Among these non-uniformities are temperature, pressure and gas composition non-uniformities, e.g., as encountered in vehicle exhaust samples.
An apparatus for temperature-independent determination of probe gas concentration has a spectrometer for employing the selected spectroscopic technique including techniques with broadband and narrowband light sources. A laser is preferably used as the narrowband source. The apparatus also has a processing unit for determining the probe and reference temperature functions such that their ratio is substantially constant over the temperature range. A computing unit is provided for deriving the probe gas concentration from the probe and reference reactions.
The apparatus additionally includes a unit for calculating the ratio of temperature functions. Furthermore, the apparatus has optics for directing the probe beam from light sources to the detectors employed by the spectrometer.
The details of the invention are explained in the below detailed description with reference to the attached drawing figures.